Radio communications system



March 2, 1954 J, c, O'BRIEN 2,671,166

RADIO COMMUNICATIONS SYSTEM Filed Jul 1. 1950 s Sheets-Sheet 1 PIC- 1. BLOCK D'IAGRAM g" TYPICAL STATION F =.1A. THIS CONTACT CLOOES LAST TBl T0 AND OPENS RELAY TR FIR5T I (HQ: B -3 cm TB 20 w ax 18 go TNE E e n IG. GENERAL ORGANIZATION SYSTEM FIXED STA. FIXED 5TA. 6 CH.N0.1 TONE A MOBILE 5 A CI'LNQZ.TUNEB Y GHNOL Tom-ab s MOBILE 5m. MOBILE am M. 7 H CHNO.1 TONEA Jul-N01 TONEA FIG... I TONE RELAY ORGANIZATION Z8 TONE RESONANT CIRCUIT Bnnentor w am,

THI5 CONTACT March 2, 1954 v J c O'BRIEN 2,671,166

RADIO COMMUNICATIONS SYSTEM Filed July 1, 1950 3 Sheets-Sheet 2 TONE F |e..4, TYPICAL fiTATlON m.51 A CARRIER 1: FREQ- TO GE I L 16 Ml uENcY MODULATED 1:: WITH VOICE MESSAGE AND AMPLITUDE M F'M POWER movuumzo wnmow;

TRANfiMlTTER AMP I CLOSES LAST 7 I AND OPENS L5 FIRST FM .1

Z B RECEIVER B l ONE g 18 TAMP 12 TR :17 fi v- Piaf). 1%: LIMITER 2 uMnER DISCRIMINATOR '2- AUDIO AMP'S. L5 OSCILLATOR MIXER 39 LF. AMP. (5+) {L 37 g 48 38 37 4a (am-H TONE REfiONANT TN CIRCUIT 3nventor Gttorneg March 2, 1954 J c O'BRIEN 2,671,166

RADIO COMMUNICATIONS SYSTEM Filed July 1, 1950 3 Sheets-Sheet 3 AUDIO AMPLI- FIER5 EIIIIF-IW a Patented Mar. 2,1954 1 UNITED S'EI'AIEfiZi meant 2,611,166 a RADIO? COMMUNICATION S SYSTEM Fifi? J ohliizfl. iofBrien; eliochester r 5N. assignor to s GeneraleRailway-fiignagltCompany, Rochester,

Application July l'y1950, Serial 'No;- 171,635,511: 5 olaimsww Cl. 250 56) .QQ

trol *ancl thetransmitter is "deenergizedz'lhe're 1'1 ceptionc oha strongnoifechannelrsigna mayejde-m spite athe strongly selective characteristics-of ith usual communications receiver, cause.-,;the:squel ch-ie tween afixed station 1 and i'nobife stations. t 'to activate the ireceiverz'output: acircuiti so: that In providing radio communications systems forf the signal from the nearby transmittemissheardm mobile stationsyit is usual toprovider what is This; spurious:rreceptiomis:highlynobiectionable known-as a squelch oircuit controlin:each.:. re 1 andzisrmostrdifiicultgto.preventain commonlm'usedre ceiver; so that duringthe =-'absence 0f a" signalt'r receivers in ,vievwof theiproximity: ofvthesmobilezre the receiver will'suppressthe r-ioise-vvhich isinreceiver to the transmitting zstatiomzonfithei. adieu: heren-tly produced when no signal is present jacentgjcha-fmeli, Such noisemaybe due either-to external radioix Thespresentiinventionrproposessitozsooorganizero noise ornoisevoltages' whmh are produe'd withi n thecfcommunication' ystemrs'that 5a: receivemwiltrn thereceiver-itselfs- When such-"a squeleh circuit respond only to the reception-1i. f ca'signai-iirom-irz control is usedrthetermination of aj'signal' from a station; transmittingironiiitsi ownrzchahneLnin a sending station results ina" so call'ed noise w thisiivayz-rthe'spurionsreceptiomwilrihepreventedm tail or a eras 'which is 'caused by the time and no interference:willttakesiplaceeoWhmthete constant of 'the circuit--'when-the*receiver=is ren transmitting :stations transmit'i'on bothtchannelszs dered ineffective. -The use of'a'squelchcontrol simultaneouslysithe?ofiechanneh'signalciordinaniiy of the"usua'l typ' 'is also'dependent upon' the will notrbe'heardievenithonghiitmaysbestronger reception ofasignal above a=predetermined=valuer than-theYdesiredrsignal*heeauseofithei'discrimina Thisfvalue must be'selected sufiiciently high to tion against it by the highly selective circuits inn: prevent "the. receptionof *strong-noisewoltages the'receiver: from opening-the receiver; 'but-*-in s'ome cases,-a Generally:ispeakingtiandcwithout'imakingnanyro Signal below t level might .b '-read 1 yhtel-l attemptzto'describeztherexactinature'iof'zthe in it; ble. if 'theflnoise-were at -a low leveln Inothe ventiomsitis proposedfto have each transmittingzi: words; the squelch"control-must beset -*to 'sup-=' station effective to transmit a tone alongewithrz press noise for t the worst conditions; but there i a, 1egitimateimessageizbutiso'i'organizemithait .this are some locationswvhere'thenoise leverisrela tone is not iappl'ied until'i'afterfithesicarrieriihass This inventioi r'relates to communications? systems; and more particularlyipertains:tosiradio 'tively low, and in suchlocations of the mova'ble 3' been 'established and is removedzl'at:theinchbf:then" receiver it is a' hardshipfor the receiver -to be messa'geqiustbefore'thefcarriemterminatem Eafihr: closed .justvbec'ause. theireceived-signalis' Weak-s receiver is then organized to have means'rfior rcnieii One. purpose vofwthe" presentxinventiofi is to" dering 'its loud speaker eifectiver to"reproduce-ta so. worganize they transmitting":- and receiving" message signal only whe'n such tone is accom-i equipment that the conventiona't squelehcircuit-g I panying the carrier signal. 1 Since the i tone is I. control .Wiilb'e. unnecessary,"'and":yeti.'each-reappIied SubseQuent to the -establisl'imen-t of -theti ceivenwill be suppressed'or closedexcept when carrier signal, the loiid speaker' is'not rendered a i im Si n l is to be'receivedrf: A1S0;an* effective to reproduce *the"message signal until other object .of theinvention is to so organize th une ta q ew e o ge this, controlrthat. .-the receiverrwilltb'ef sensitiv'e*- loud speaker -is rendered"ineffectivebefrwbhe to aenale l n as itlis rec ived r eardlesss f carrier signal'teases" so that no noise tails cam theliexisting noise levels. beproduced-bythe-loud speakerf In. mobile communications asystemsntwoi sta-J; Thisrorganizationzis. also 'efle'ctive"to prevent tionsw-maylordinarilyrlcommunicate readilyf'with' the 'reception of any-signal froin anadjoiii'ringci each Other Withoutinterfer-ncerfmmfimadimn: 45 channel because such signal willinot containthe ingichannel as. long ,asrtheir; spacer-relationships tone required to open theioutput of thetreceiverr' areifavorableh At times however, asi.,when.a However all thestations"'belonging'.toa-"group* receiving station for wonemchannelis adjacent the" having means responsive to the same ltonetofor transmitting-1 :stationjoflanother channel anrolfra givenjcarrier' channel are-"held incommunica-l channel signal from the nearby transmittenmamao tionwith ;the.' station then.transmitting"gn:that proyide..,a very; strongl-iinputnsignalnto th'elfrechanneLandmithithatfltone; In"".this "way,wj CBiVfilfs'; :Uncter ,-the-, -=usua1:-.-operatingezconditions a theistationsnof. a"group are kept informed; Wit QaEhTS5J&tiOIl;=I .S mostr-iof-thei times-icontrolledgiorio respect to Ltheirrown group it Thisiprinciple ,;ca stanrihymperatien; i; e.,-,.its,- receive]; is, energizedw begexpandedgif iclsiize'd, .so. thatlithei;mobile.llsta but-Elias :its; output muted bywthe squelch con-.455 i'tionssassociated with .theffcentral .station Imay;

be divided into two or more separate groups for the reception of separate classes of messages and yet provide complete facility of intercommunication between the stations of each group. Under certain conditions as in areas where the desired signal is weak but the noise disturbances are strong, the tone may be satisfactorily received even though the noise appearing in the receiver output is so great as to make the desired signal unintelligible. Accordingly, the organization of one embodiment of this invention has been so devised that the squelch circuit control is still effective to mute the frequency-modulation receiver output even when the tone is being received.

Other objects, purposes and characteristic features of the present invention will be in part obvious from the accompanying drawings, and in part pointed out as the description progresses.

Fig. 1 illustrates in block diagram form the 0 general organization of a typical station constructed in accordance with the present invention to use an audio tone above the voice range of frequencies;

Fig. 1A illustrates a modification of part of the organization of Fig. 1;

Fig. 2 illustrates in block form the general organization of a radio communication system involving one or more fixed stations and one or more mobile stations;

Fig. 3 illustrates diagrammatically the circuit organization of the tone portion of a receiver as used in Fig. l and constructed in accordance with the present invention;

Fig. 4 illustrates in block diagram form the general organization of a typical station con structed in accordance with the present invention to employ frequency modulated carrier signals for the transmission of messages and at the same time amplitude modulating such carrier signals with a relatively low tone frequency; and.

Fig. 5 illustrates diagrammatically the circuit organization of the receiver employed in Fig. 4 with more particular emphasis placed on the details of the tone detection ganization.

Fig. 6 shows the circuit organization of a frequency-modulation receiver. The reception of tone and the operation of an associated squelch control circuit are both effective to mute the receiver output.

For the purpose of simplifying the illustration and facilitating in the explanation, the various parts and circuits constituting the embodiment of the invention have been shown diagrammatically and certain conventional illustrations have been employed, the drawings having been made more with the purpose in mind of making it easy to understand the principles and mode of operation than with the idea of illustrating the specific construction and arrangement of parts that would be employed in practice. Thus, the various devices are illustrated in a conventional manner and various conventional symbols are used to indicate the various devices, wiring connections, battery, or other sources of electrical current, and the like.

With reference to Fig. 2, it is assumed that two fixed stations, stations 5 and 5, are located at some distance from each other. Station 5 is considered to operate on a radio frequency channel No. l and employs a tone A in connection with the transmission of its messages as will be later described in greater detail. Associated with this station 5 is a number of mobile stations,

portion of the orsuchjas stations I and 8. These mobile stations have equipment exactly as employed at the fixed station 5 for transmitting and receiving on the radio frequency channel No. i, and these stations also transmit tone A along with their messages. Fixed station 6 is arranged to transmit and receive on radio frequency channel No. 2 employing tone B. A number of mobile stations of like structure for the same channel and using the same tone B may be associated with the fixed station, such as mobile station 9.

The radio transmitters and receivers at the various stations or locations are assumed to be of the same general type, i. e., amplitude modulation type; frequency modulation type; or the like. Also, the equipment for each group of stations is adjusted to operate on the particular frequency channel designated for that group, and is also arranged to employ the particular tone selected for that group. In connection with the first form of the invention, the selected tone is assumed to be applied to the audio sections of the transmitters and to be received in the audio sections of the receivers. A block diagram of a typical one of these stations has been shown in Fig. 1.

Referring to Fig. 1, it will be seen that a radio transmitter and radio receiver are associated with the usual antenna A and a battery 13 which supplies the anode potential for the various tubes of the receiver and transmitter, as selected through the contacts of a transmit relay TR. The antenna A and plate supplying battery B are normally connected to the receiver through back contacts It and II, respectively, of the transmit relay TR which is normally deenergized. Thus, the receiver is normally in condition to receive, but associated with the receiver is a tone responsive relay TN controlled from the input to the receiver through a tone amplifier i2. The loud-speaker LS is associated with the receiver and its circuit connections include a tone filter i3 and a back contact M of the tone relay TN. Thus, the loudspeaker is normally discon nected although the receiver is ready for the reception of a signal. When the signal for its particular channel, such as channel No. l, is received accompanied by a tone, the tone relay TN is deenergized closing back contact it so that the loud-speaker LS is rendered effective to reproduce the voice message accompanying the carrier signal. As previously mentioned, this tone is not received until after the carrier signal has become established thus eliminating the reception of any noise incident to the establishment of the carrier signal. The tone amplifier !2, of course, facilitates the control of the tone relay as will be described more i detail hereinafter, it being sunicient for the present to know that this tone amplifier is so constructed that the tone relay TN is deenergized only when the particular tone selected for the associated channel is present on the carrier signal. The tone filter i3 is included in the loud-speaker circuit so as to avoid any stray components of the tone from being reproduced by the loud-speaker.

The radio transmitter has associated therewith the usual microphone M and a tone generator [5. This tone generator i6 is normally inactive along with the inactive condition of the transmitter.

In this connection, it should be noted that the form of the invention illustrated in Figs. 1 and 3 is assumed to employ a tone of a frequency somewhat higher than the usual voice frequencies, which tone is impressed onto the usual audio ravines channekse Ofi-I and; reeetverrt example; freqttenciesebetweenewm and; 3I10I1Lma5 be usedfor the rasigesotevoica freq-uenciessinsucks afsystem, so;thattartoneof any sn-itablerfrear ency such 436:? 350mm; above i-mighta-welh be employed fortherzcontroletone features herem-zdisclosedm It ism-01 .1'comsecapnreciateda that? the: greater: the difference; betweenrtheetone. frequencyi ands-the voice tfrequenciesathezeasierritz isttozdiscriminate between =themtby; thesuseeof .filtcrseandrth'elikeass-shown'inrdetaihin:connec-tionawithfiigi,3,

When iteiszlesireditottransmit a'rsignahatranse mittbuttomzzTBeissactuated the-operator, and the. closures ofibaeke contact I? of; thiec-h-uttoir energizesathe-transmitareiayJER:through arr-pt viorus;circvrite- Thee openinarofi bacle contacts: I9 and. I I rendersetheereceiveryinaetiveawhileythe closurezof'tfrontscontactss I (It-and; I I v rendersrthe transmitteravaalablerfor 'operation..h 'Ihe-ztransa mitt button lTBz hassav contact,v I8; which-:closes siightlyyaften: theif closure oft contacts I 1:: topcon nect .1 toetheetoneegeneratortI scandsthereby renders it actives In this way,-, each: time the operator: aetuatessthe: transmit nbutton T3, thetone; generation; iss renderedr active: just slightly aftersthetransmsitter:hasbeen rendered reflective. At each ofstheestationsebeing;transmitted;to; the reception pf tonenust subsequenttto .the reception of -.carrier prevents :the loudspeakers :attthosestae tionssfromzbeing energized zuntil: the: carrier 7118.3 1

beene established; Gontacte I8 also; opens; just prior; to:- the opening; of contact- I when, the operator releasesmheatransrmtbutton: I8. atathe endsof hisemessageu Thisediiierencetinzcontact opening; time: meanszthat-,- ther tone generator cease its operatiomjustr prion to; the;=1:endering inactiveiofrthedzransmitter;t Theremovazl g-Of tone caused-thy; the-openings of..contact-:- I8 energizes thBJJINE relay; ateach 10f the stations I to i transmission has..-just-.heenamadey,and aattert a n brief I interval; correspondmg; to themickup t'me oft this: relay; T-N,, each; receiver loudespeaker; is deenergized... Therefore,- ;at .thetransmitteaylhca: tion, theeremoval of a carrier. caused.-:byg-thee (trope ping; aWayrOfJreIav. TR does snot occur until-the tonezhaszzbeen tremoved for; a; suificient-Jength 10f time .tot-permitethe TN relayt at each: the:re+ maining-- stationsa' toz pick; up, and.- thereby; def.- energize, the 'loudespeaker. In; other word's-,- the timecprovidedlforr relay- Tfi'tmdrop arway cone w sisting of the time intervalrbetweenttherrespeee tiveopening; of; contacts I 8 and I 1.-plus-:the.-drop away; time, must exceed the; pick-up time? of relay;-

It may bea:in.-.somel-casescdesirableetcahavepa longer intervait between... the' establishment; of transmission and'the applicationwoiithertone: Thi&- can. be accomplishedeither. by mechanical meansassociated ,witii.the. .contactsv IJTandA I8iof theztransmit button. TB, :or. electricallmeanamay c bee provided; the provision of; at slowracting relay 'ItGl'asshownin- Eig, 1A. It israssrm'ledsthat thetcontactjl'B.ofiEigalrmay: betreplacedibyythe transmit button: TBJ of; Eiga. 1A.. ina whicha the contacts; I9: andrmecloset-at ltheasametime. The

contact I 9 supplies-z energry when 2 closed, toithe transmit irelayf-TRthepames-as contactal Lois Eig. 1; The. closureof .contact :20zenergizesstheslowe acting; relay; 120, which is; slightl rslcveto) pick up; The; contact: 2-1adoess'notcloseeuntil attea-v a briefl. intervak. Whemthisszcontactelshiseclosed, itaappliesc +9 :tozthestoneeenerator; I fistheesame Atrtheaemn organ trflmmissionathezreieaseeciff W the transmit button TBI opens contacts I9 and of; tone. ircuitzand,for:thissreasonetheebysnassrcondenser 6 20" simultaneousli'; ctreniteforitheelhm ting toneegpneratcrvcantrol relaygllfcawhichimmediatelytdfonseaway opening frontecontart; 2.1, and'i'removing the; energyjiiom the-tonetgeneraton I61, Also aslsoorrassrelay :TC isedroppediaway, theie rgizing. .circuitlfor relay, TRttl-iatehasibeen maintaihedlthrongh .frontpon taactt ukofirelaxc Ticg'iss opened; Therefore, after aib'rieflintervahporresppndihgi the release. time. oti'relay 'I'R; this, relay idfopsaway andlclosesits. back contacts Land; I I Iiieflremoval'. ofcltone. caused byrtheidropping; awaxpoffrelay; 'ICZZener: sizes the TNwjf relay; atveachrofthe :other; stations. After an;intervaL.equalitopthepickupjime ,ofgthe 'IENereIay these; re1ays ,pickup,',thereby deienergiz: mg; the --respective. receiver loudespeakersi A'n.-, otherrconsequenceiotitlie dropping amay; o"relay- TCisstheideeneegizationroffrelaygm which, ,when it drops; away.v removes carrier.) The time... at whichrelay-13R. dropsawa gmustith'us occur after, piching unoizLtheilbLrelay at eaclipfthe stations sarthate-thel. loude'speaker; at each.- of .i these, star tionsewill a bawinactivecwhencarrien is; remored. Ini other; words, theerelease: time; provided; for.

- relay TR'i-mustibeegreater; thanithelpicliup time of relay 'llN'a ReierringtoaFig l3i theereceiizer.andjitscorganie zationpflmgi lciashownin-lgreatendetail; 'Ifh'e. ontptuttppwer amplifier is assumeditc b.e;oijthe' pushepulhtype andeprovidsanoutput'to.a, center tap transformen'I'Fthrough-tvhich. (B;+) is sup.- pliedtmtheEpJate; circuits of this-audiot-amplifier. Thetsecondary oflthistransformerlTE has aflcon: nectionsthrough.theztoneefilten I 3'Jwhich includes anrinductance and -coi-id-msen1having such .values ante-prevent; th'evpas'saga otlthe ;tone,for which thiststation is designedl circuit-lincludes the back contact I 4' of 'the tone relay, previously explained.

The primary of'theaitanstormerfI'Frhas by-pass condensers 23. shunted across the primary for lav-passing: thEftOIIEfi'EqIlEIICY which" issupplied to'fa"toneirespnantcircuit'including*a transiormer 25"havingpriinamand"secondary windings: The primary-"winding ofthis: transformer-25' has a tuning;pondnser"ztfconnected 'across it, and ithe secondaryiwindin'ghas a'pptentiometerconnected across it; 'Iifepotentiometerserves as a level or volume control forzthleinput'lto'tubei 8"having its controi grid iconnected through" condenser 29" to themovabiatap ofthepotentiometer; The;controligridfis. alsorsuppliediwith the grid 1eak'..re'- sistor; 302" mic output. circuitioffthe' tube 23 in cludes; the; tone relax TNand a suitable plate sourceeofienerg-y, such; asfiattery PS 'Which may be'ithe samasourcewashatteryBi Aby-pass condnsenil lisl sh'untdacross (the tone relay TN.

Trier/grid lalf' input" circuit for; the tone amplifier tube 2"3"is' so adjustedth'at'when no signal is .present,, thewcontrol .grid tor; the tube. is-iust siightlitinegatiizaandlthius 1am output is; supplied tcgtfiewtoneerelasr maintainine, itrno malfiz ner: eized; Blower/en,whema signallisleceivedt the ccmlnserlfliandigridleamfl cooperate. to cause-a negative;bias;to;.be;bi1iltlup;on,the control grid, thuswr dneing the, aweragecoutputl of. the plate ammrmxmaze; and,' .f.6r;this' reason the tone relay, TN becomes deenergiz'ed and; drops away closing;hackmontzuzti I Alto. allowx theioudspeaker Ls ta. reproduceeany, message, that is received.

Itewnlebeeappreciated;,oafgcourseathat this. type ofe'controlJor-wuhcMB; ailowsarertainaamcunt rec an: to; appear in, the. output 3| is provided so that'this' frequency will not Th; contact 20* opens aevmce affect the tone relay TN which is highly inductive.

Referring to Fig. 2, it will be clearly understood that this mobile station 1 can readily communicate with the fixed station 5 by each operator taking his turn transmitting and receiving in the usual way. This operation, as above described, is such that the loud-speakers of the respective stations are not rendered effective until the carrier is established and they are rendered ineffective before the carrier ceases and in this way prevents the reproduction of noise while no carrier signal is present. This organization allows the mobile station 1 to move away from the fixed station 5 and continue to have communication even in the fringe areas of reception. This is because in distant locations there may be but little noise and a very weak signal may be received and still be intelligible. However, should a mobile station, such as station 8, move to a position adjacent one or more other stations, such as stations 6 and 9, it is readily apparent that the strength of the signals from these adjacent stations on the adjacent channel No. 2 would be of such a relatively high value as to tend to overcome the discriminatory effect of the tuned circuits and thus crash through to cause spurious reception if it were not for the features of the present invention. The reception of a carrier signal for channel No. 2 does not open the recei'ving organization of station No. 8 because the tone B does not operate the tone relay of such station. But when a signal is received from station No. 5 accompanied by tone A, the receiver of station No. 8 is opened and proper reception can be had. Obviously, the transmission from station 8 does not affect the receivers of stations 5 and 9 because a different tone is required for those stations.

Modified forms of the invention It is preferable in communication systems of the type used for mobile transmitters and vre ceivers to employ frequency or phase modulation of the carrier wave which may high range of frequencies, as in the order of several hundred megacycles. However, when such high carrier frequencies are employed, it is the practice to allot a relatively narrow channel for each such system with a relatively small portion of the frequency spectrum left free between each of the channels. When a tone frequency is added to the usual voice modulating signal the resulting output signal covers a wider band of frequencies than is allotted to the channel for such system. For this reason, it is highly. desirable to employ an organization in which the use of the tone frequency does not materially change the band of frequencies used by the system.

With this in mind, it is proposed in accordance with this form of the invention to transmit the voice messages by frequency or phase modulation of the carrier wave butat the same time to amplitude modulate the carrier. wave in accordance with the tone frequency which may be relatively low such as in the order of one hundred cycles. In such a case, the output of the transmitter may be amplitude modulated and not have the band width of the signal cover more than two hundred additional cycles. Obviously, an increase in the band width of two hundred ycles when the original band width covers thousands of kilocycles does not materially reduce the'margin between that channel and the adjacent chanbe in a relatively nels on either side. It is of course understood that one hundred cycles is mentioned merely by way of example, and that any suitable relatively low frequency may be employed.

Referring to Fig. 4, it will be noted that the general organization of a typical field station is very similar to that shown in Fig. 1. One distinction is illustrated by showing an FM transmitter as including a power amplifier which has an input to its grid supplied from the tone generator 6 to thereby amplitude modulate the frequency modulated signal supplied to it from the earlier stages of the transmitter. It is believed that this structure of such a transmitter will be well understood to those skilled in the art. Although an FM transmitter and FM receiver have been shown in the drawings, a phase modulation transmitter and receiver could instead be used.

The tone generator [5 is controlled by the contact (8 of the transmit button TB the same as described in connection with Fig. 1. Likewise, the contact H of the transmit button TB controls the energization of the transmit relay TR. The contacts H and ill have the same adjustment as described in connection with Fig. 1 so as to provide an interval of time in which the carrier wave may be stabilized before rendering the transmitter operative for adding the tone modulation of the carrier. Also, at the end of such message, the tone frequency is removed first so that the receiver at the receiving station is rendered ineffective to give an output prior to the cessation of the carrier signal. In this connection, it should be understood that if added time intervals are required, the control shown in Fig. 1A may be applied to this Fig. 4 the same as described in connection with Fig. l. It will also be noted that since the tone frequency is not injected into the audio channel of the radio receiver, there is no necessity for a tone filter in the loud-speaker input circuit. The tone relay TN is controlled in response to the tone frequency through the tone amplifier I2 the same as previously discussed, but the input through the tone amplifier is supplied in a special manner from the first limiter stage of the FM receiver as more particularly shown in detail in Fig. 5. It is sufficient for the moment to understand that when the tone is received, it affects the deenergization of the tone relay TN and closes back contact 48 to allow the loud-speaker L5 to reproduce the voice message.

The FM receiver indicated in Fig. 5, has several component parts including radio frequency stages, oscillator, miver, intermediate frequency stages, first limiter, second limiter, discriminator, and audio amplifiers for supplying an output to the loudspeaker LS.

It is well understood that the first and second limiter stages are employed in an FM receiving organization to remove variations in amplitude of the input signal. This is because noise is partially represented by amplitude variations and these limiter stages in removing these amplitude vari ations remove a major source of the receiver output noise. Since the tone frequency in this form of the invention is an amplitude modulation of the carrier wave, it is desirable that such amplitude modulation will be a relatively low percentage, such as in the order of 10 to 30 per cent. The reason for this low degree of modulation is that it is desirable that all of this amplitude 1nodulation be removed by the limiter stages, and yet sufficient signal must be permitted to pass the limiters toprovide proper discrimination of the 9 frequency variations togive a iaithful reproduction diithevoicehie's'sage. The firsumrerstqge of an reeiver d"veldps avoltage at its oontrol gridw'hic'h Whenb both "radio andfaufdi'o I plies direct current voltage ieh var ies in a'e'brdancew'ith' thestrength of the iiiputsignal. fl his voltage is jcommonlfy -"k'nolivn as an autism fie volume control voltage landfal This AVC bus strewn 'inj'F I voltage from" the controljgrid of the first: v tube, 3 5 through the "resistor 36 and""ov'er thebus 3! to the early staeesbf meteeeiv ana 'lso through resistor 43to'theg'rid"o a ifier 4l. The'AVCbusUisby-p n'd for both radio and audio fretluencies by Com denser 38. Q g n I Also, the "input signal "to"the 'fi 'rst linfter has componentsoftheaiidio' reqlie 'ywhieh may'te supplied "through resists;- 39 and doubling denser to "to t e contrblgrid tirtnetqn' fier tube '4l. This ammingeonaaasmn'i surfieie'ntly large to allow thefrjee pajssagebf tlie to'iie frequency. The resistors "'36 "anu aa ag ms decoupling resistors. A'lso, aedndensr '42 re rovided to by-pass any radiofrequencies t This tone ar'fiplifiertfib'e 4f "preferably operated as Class A with a'self b1as1ng resist6r 44 'bypassed 'for "audio frequencie by 'eonden's'er H5. The screen'gri'd'is supplied with (3+) tl'irou'gh a Suitable resistor 46"tvlii'h Li's by paSsed for audio frequency by oondnser 4'1.

As the strength of thesignaryane's at the input to the first limiter, "the amplitude T of the tone signal applied between'cerin'ergne and'c at-h'o'deof tube 4| varies orrespindinlgl'y. I't is desirable, however, when a tone sighal -ls present that the amplitude oi the'tone appearing in-the-outputeircuit of tube 4' I and appliedto the tone-resonant ciruit be at a relatively c zfnstant a'n iplitude. Therefore, tube 4 ss l e s'i eh jeh 9 be a variable mu tube so thatgtihe yariationfof grid bias produced by th'e nl/fc fvoltage Waries' the re m llm ner qe ir d... ih ere thesignal strength at the -controlggrid 9f the-first limiter tube 4 I increases, for example, iane-ampli etude of the tone voltage at the -control;grid --of tube '41 -also;inereases. -The accompanying-increase -of -AVC voltage, however produeee a more negative bias voltage, thereby lowering the gain of the amplifier including tube "41 so that'itsoutput tends to remain at a substantially :eonstant amplitude. The applieationof the-AVG -to-other amplifier stages in thereceiversueh-as-the intermediate-frequencyamplifiers forexamhleutendS to-maintain the-signal level aft-substantially constant amplitude, thus --prevent-ing overloading of these amplifiers at high signal-levels'with attendant loss of selectivity.

Theoutput-ofthe tone amplifiertu-be issupplied to the-tone-resonant-circuitinvolvingtransformer 25 and tuning condenser -26the-same as discussed in detail in connection with sFig. -3. Obviously, the oondenser 26*wil1 haveto be ofa suitable "value to -corres'pond-to the resonant frequency of the -'transformer=eondenser combination for the relatively low tone frequeney now enn ployed "to amplitude rnodulat'e the earrier wave. The-tone amplifier tube 41 suppliess'uitablednbut to the tone "resonant circuit and the iebfoper volumecan th'en 'be' supplieil to the-' control tube 28 by the adjustabletap resistor r s'o that 'dnen the carrier signal "is dnodula'te'd with the rla tively =-1bw tone freq-uencyfi nontroi wet as will berenderedoperativee are1etive1y4ew 1eve1 by reason of the increased bias produced from the condehser grid leak combination (including-eon- "denser' Z Q -a'nd grief leak-resistor ee) Norrnally the control tube 28 is 6peratingfwith itsoorltrol grid just slightly lig'ative "and "slifiioijeht 'plate current fiows t'o"allowthe tone relay TN to be normally energized, but thei'p'resen'ce': of'the tone freqlieney negatively biases this oontrol"tu'b'e -28 and reduces the averag output or the tube 28 causing thePtone' relay TN to'drop'aWay the same as 'describe'd cementum with Fig. =3. The closure oi -its bali Contact 48 applies (-3 from a s'iiitable plate-source to thein'idtap "of the eonpiing transformer ES so that the power amplifier 'o'an pr'ovide an odtput' some coupling transformer 4ft for rndering the loud-speaker Ls tap-able of reproduin gtlie voice message.

this way. the amplitude "modulation bf the signalis employed to providesquelohbontrm for the frequeney' or' phase "modulation portionbf the signal; the "same as previouslydescribed 1h*-'c6n-' 'fiction with 'Figs. 1 aiid'3. "Also, this modified fornfof tlie' invntion provides the same features of oner'ationas previously described ineennecnon 'With'-Fig.- 2 and the first" form.

v In both forms of theinve ntion, the tonerelay TN is shown as b'eing normally energized. This is sothat'any failure in its eontrol 01mm; fvvill result in the"deehergizatiofiof flit-2;relay to render the receiver active. For *eiiample, if tube 28 should "become burned outfthe 'tone relay ;N would "-dro'p away "and the receiver would still operate. This arrangement 'is more desirable than a normally deenrgized'eireuit'arraiigei since a failure ofthis tipe is "more comma han an internals'hort cirouit in'the fltub'e whlcllfih the case of a nori-nally oleenergized"arrangement, will nonestore the reeiverto the listening condition. However/the tone rela'y oouldbe'ndrmally deenergizeol and the reception'of thee'entrol tone .be employed to render it"active'or pifid upfif desired. This latter arrangemeh er'ebntrol would be; inore economical fr'o'r'n the standpoint of bower Consumption, but in systems "-of this type the 'reliability -feature 'usn'alliz is the predominant 'one. Y

As has been mentioned.mobile's'tatidns niay at times operate inaras wherereceived signalsare weak but interfering noise is strong. At {such times, the tone signal ma'y be suitably rec'eived even though the "signal that would -lf1'e'ard in the "reoeiver' but'put is unintelligible. The r1 bodi'inent of the invention shown "in B g. 6" lustrates a circuit orgf'anizationwherein' the r 6pt1on of tone controls the activation'of'there put may be muted even w'hen the tone'sign'alis received.

The general organization of this embodim'e tie for the mostip'ar't similar to'thatsh'ovvn'in' 1: form in Fig. 4. Thus, the tone frequency is preferably transmitted as an amplitude mooliilat'ion of a frequency modulated Carrier Wave; One point of" difference 116 ,however, in; the do relay TN and. ais'o 'tvitnrespeotto the mm which this relay "controls the receiver engine will; be; "ore fully ekl lained.

The mrree ivf shown in Fi'gfe inc ude .j'sl r p g nal tb the radio frequency amp ifiers, a latpr. -r'niger, termedi'a'te frequ'enoy fmm first and estate "-limi ters; a a

amplifiers, and a loud-speaker LS. Also included are a tone resonant circuit, noise amplifier, noise rectifier, and squelch control.

The first limiter stage shown in detail in Fig. 6 is similar to the corresponding first limiter shown in Fig. 5. Thus, an AVG voltage is provided on wire 50 and applied to the intermediate frequency amplifiers for the purposes already explained in connection with Fig. 5. In addition, a tone frequency voltage appears on the wire and is applied to the tone resonant circuit. This tone resonant circuit includes two tuned circuits each resonant to the tone frequency. The secondary tuned circuit is shunted by a rectifier 52 and resistor 53 connected in series. The resistor 53 is shunted by condenser 54 which is of sufficient size to filter the rectified tone-frequency voltage appearing across resistor 53 as indicated by the symbol designating the rectifier 52. The polarity of this rectified voltage appearing across resistor 53 is such as to make the upper terminal of this resistor negative with respect to the bottom terminal whenever the tone signal is applied from the first limiter to the primary tuned circuit of the tone resonant circuit.

The discriminator shown in detail in Fig. 6 is of the center-tuned type, the operation of which is fully explained in my prior application, No. 47,458, filed September 2, 1948, now Pat. No. 2,644,084, dated June 30, 1953. As is also explained in this prior application, amplitude variations of the discriminator input signal which might appear as noise in the receiver output cause an unbalance in the plate currents of the two electron discharge tubes included in the discriminator. This unbalance. of plate currents then produces corresponding variations in potential across the resistor 16, and these variations in cathode potential then act degeneratively with respect to the amplification of these tubes, thereby tending to suppress these amplitude variations.

The alternating voltage appearing across resistor 10, therefore, is proportional to the noise which has escaped the limiter stages and would otherwise appear in the loud-speaker LS. This alternating voltage is then applied through coupling condenser 55 to the control grid of a noise amplifier tube 56. The parallel tuned circuit including condenser 5'1 and inductance 58 connected between control grid and ground or tube 56 is resonant to a frequency above the range of voice frequencies appearing in the receiver output as, for example, kc. per second. Since this parallel tuned circuit is resonant only to a particular frequency, it presents a high impedance only to those frequencies and other noise frequencies are shunted to ground and do not appear between the control grid and cathode of tube 55. Since the noise is generally distributed rather evenly over all frequencies, the particular frequency of noise voltage applied to th input of tube 55 is representative of the noise voltage amplitude. By selecting the representative noise voltage to be at a frequency above the voice frequency range, this voltage will be substantially unafiected by discriminator unbalance caused by voice signals.

Grid bias for tube 56 is provided by a cathode resistor 59 which is by-passed for the frequency to be amplified by condenser 60. Screen grid potential is applied from 13+, and the screen is by-passed to ground for the frequencies to be amplified by condenser GI. Plate potential is applied from B+ through the load resistor 62. The amplified noise voltage appearing at the plate of tube 55 is applied through coupling condenser 53 to the plate of diode 64 which has included in its plate-cathode circuit the resistor E5. When the instantaneous polarity of the voltage appearing between the plate and cathode of diode 64 is positive, this tube conducts and its plate is then at a very low potential with respect to ground. When the voltage between plate and cathode of tube 64 is negative, however, the diode cannot conduct so that the voltage at its plate becomes correspondingly negative with respect to ground potential. The negative rectified noise voltage appearing at the plate of tube 64 when noise is present is applied through decoupling resistor 36 to the upper terminal of resistor 53 and is filtered by the condensers 5d and H which bypass the audio frequencies to ground. Thus, it can be seen that in the circuit from the control grid of squelch control tube 6'! through resistor 53, resistor 36, and resistor 65, to ground, two direct voltages are applied in series. In this series circuit, these direct voltages are in opposition, i. e. they are of opposite polarity so that the grid voltage of tube 51 varies according to the difference in amplitude of these two direct voltages.

The squelch control tube has included in its plate-cathode circuit the winding of relay TN so that when this tube conducts plate current, relay TN is energized, thereby closing its front contact 63 to apply (B+) to the audio amplifiers. The cathode of tube E1 is connected to a variable tap on potentiometer 69. Since the opposite terminals of this potentiometer are connected between (13+) and ground, the bias voltage on tube 61 may be varied as desired by movement of the tap on this potentiometer. To obtain the desired operation of this circuit organization, the bias of tube 61 is adjusted so that when the direct voltage across resistor 65 is of low amplitude corresponding to a low noise level and no tone signal is received so that no direct voltage is applied across resistor 53, tube 6! will not con duct sufficient current to pick up relay TN. In other words, even when the combination of signal strength and noise level is such that the noise that would otherwise appear in the receiver output is at a low level, the squelch control tube 6'! will not conduct enough plate current to pick up relay TN to activate the receiver output circuit because the necessary tone signal is not then being received. If when the tone is received with the noise still at the same low level, the direct positive voltage across resistor 53 increases, thereby also increasing the grid-cathode voltage of tube 61 sufiiciently so that the increased plate current of this tube allows relay TN to pick up. However, if the tone is received but the noise level is of such a magnitude that a large negative direct voltage appears across resistor 65, the voltage applied to the grid of tube 61 decreases, causing relay TN to drop away and remove (3+) from the receiver output circuit. In this way, the operation of relay TN is dependent not only upon reception of the desired tone signal but also upon suitably favorable conditions with respect to the noise that would otherwise appear in the receiver output.

Having thus described two specific forms embodying my invention, it is desired to be understood that these forms are selected to facilitate in the disclosure of the invention rather than to limit the number of forms which it may assume; and it is to be further understood that various modifications, adaptations and alterations may 1 be applied to the specific form shown to meet the requirements of practice, without in any manner departing rrom thespirit or scope oi the present invention.

What I claim is: 1, In a radio communications system, a transmitter at one station -for-transm-itting a carrier wave selectively modulated according to the intelligence to be transmitted and including means for also modulating said carrierwith a steady inaudible tone frequency, manuallyoperable switching means controlling said transmitter and having two electrically independent contacts operated in response to the same manual actuation; the first of said contacts closing just subsequent to the closing of the second of said contacts and opening just prior to the opening of said second contact, circuit means including said second contact for energizing said transmitter to transmit said carrier wave, circuit means ineluding said first contact for causing said carrier to be modulated by said tone, a normally energized receiver at another station tuned to said carrier and including an audio-frequency output circuit, a normally deenergized loud-speaker, tone-responsive means including an electromagnetic relay connected to said audio output circuit and actuated in response to the reception of said tone, circuit means governed by said toneresponsive relay for causing said loud-speaker to be actuated by said audio output circuit when said tone-responsive relay is actuated, and filtering means for preventing said tone from being reproduced by said loud-speaker, whereby radio signals received by said receiver not modulated by said tone frequency do not cause energization of said loud-speaker, and whereby noise associated with the application and removal of said carrier is not heard in said loud-speaker.

2. A radio communications system comprising, a transmitter at one location for transmitting a selectively voice-frequency modulated carrier wave and including means for also selectively modulating said carrier wave with a steady tone having a frequency different from said voice frequencies, manually operable switching means for simultaneously energizing upon closure of the contacts thereof a quick pickup transmitting relay and a slow pickup tone control relay, circuit means responsive to the picked up condition of said transmitting relay for causing said transmitter to transmit a carrier wave, circuit means responsive to the picked u condition of said tone control relay to modulate said carrier wave with a steady tone frequency, a receiver at another location including a sound reproducer and means for energizing said sound reproducer with said transmitted voice signal, circuit means including a tone relay associated with said receiver and normally energized to maintain said sound reproducer deenergized, said transmitting relay associated with said transmitter having a longer release time than the pickup time of said tone relay associated with said receiver, whereby closure of said contacts of said switching means causes said said contacts causes said tone to be removed from said carrier a sufiicient length of time prior to the removal of said plurality of stations and comprising, a transmitter at one location with means for transmitting M a carrier wave frequencygmodulated with .the-in-. telligence to lee-transmitted andamplitude. modue lated with a distinctive tone frequency, a receiver at another location including, vmeans responsive to said frequency modulationforaenergizing a loud-speakeraccording to said transmittedjintelligence; a discriminator having an un-bypassed cathode resistor for providing a first alternating voltage proportional to. the noise appear.- ingin said loud-speakena.grid-leakwbiased amplitudelimiter, circuit means connected to the control gridof an electron-discharge tube ineluded-in said limiter for providing a secondalternating voltage proportional to the amplitude variations of said carrier, resonant circuit means tunedto said stone frequency and an associated rectifier havingsaid second alternating voltage applied thereto for providing a direct voltage later for modulating inaudible tone frequency, manually operable switching means for to the recept1on of said tone frequency, and circuit means governed by said tone responsive relay is received for rendering said including a center tuned discriminator comprising a pair of electron tubes having a common cathode biasing resistor across which noise voltages present in said audio output circuit are developed, means coupling said resistor to said squelch tube input grid control circuit for producing a negative voltage in said grid control circuit proportional to the amplitude of said noise voltages, and other circuit means including a filter and rectifier for producing a positive voltage in said grid control circuit proportional to the amplitude of the received tone frequency, whereby said relay is energized by said control tube to render said loudspeaker effective only when a tone frequency is received and the amplitude of the noise voltages is relatively lower.

JOHN C. OBRIEN.

enigma Referenc UNITED STATES PATENTS Number 5 2,187,603 2,321,651 2,395,738 2,479,305 2,546,987

Number Name Date Hall Jan. 16, 1940 Caraway, Jr June 15, 1943 Hanson Feb. 26, 1946 Brown Aug. 16, 1949 Eannarino Apr. 3, 1951 Noble Apr. 3, 1951 FOREIGN PATENTS Country Date Great Britain Oct. 26, 1948 

